R&D Extruder Developments for the Wendelstein 7-X Continuous Pellet Fueling System

Abstract

Fabrication of the continuous pellet fueling system (CPFS) for long pulse operation of the Wendelstein 7-X (W7-X) stellarator is currently underway. Continuous fueling is accomplished by cutting pellets from the cross section of a continuous solid extrusion of desublimated fuel gas from a cryogenic twin-screw extruder coupled to a gas gun cutter mechanism supplied by the Oak Ridge National Laboratory (ORNL). The cryogenic cooling is provided by three Gifford-McMahon-type cryocoolers with a fixed cooling capacity. To maximize the pellet injection rate, the system heat loads must be minimized to allow the remaining cooling capacity to be used to desublimate the fuel gas. Viscous heating, caused by the shearing of the solid extrusion as it is forced through the screws and pumped through the nozzle, is the largest heat load uncertainty. A research and development extruder has been fabricated and tested at ORNL to reduce the viscous heating and verify the maximum achievable extrusion rate. This article will present the research and development of the extruder design, various screw and nozzle designs, and test results.